The present invention relates to industrial controllers for the real-time control of industrial processes, and in particular, to an industrial controller having modular components including modular input and output circuits. Industrial controllers are special purpose computers used for controlling industrial processes for manufacturing equipment. Under the direction of a stored program, the industrial controller examines a series of inputs reflecting the status of a controlled process or controlled equipment, and changes outputs effecting control of the process or equipment. In the simplest case, the inputs and outputs are binary, i.e., "ON" or "OFF"; however, analog inputs and outputs taking on a continuous range of values and multiword digital values are also used. The signals received by the industrial controller from the controlled process, and transmitted from the industrial controller to the controlled process, are normally passed through one or more input/output (I/O) modules which serve as electrical interfaces between the controller and the process. In a typical industrial controller, a central computer-like processor communicates with a number of these separate I/O modules, some of which may be spatially remote from the processor and connected to the processor by means of a communication network attached to an adapter which communicates with the I/O modules.
By dividing the function of the industrial controller among the central processor and a number of separate I/O modules, I/O data may be efficiently collected and disseminated at spatially separated points in the controlled process. The use of separate I/O modules also permits the industrial controller to be flexibly configured to meet a given control task without the need to purchase and configure unnecessary I/O modules. Modularity is provided by connecting the various modules to a common backplane which serves to conduct data between the modules. Then additional points of interface between the controller and processor are needed, new I/O modules may be connected to the backplane.
The backplane may transfer data over a number of parallel data conductors as synchronized to a clock signal also on the backplane. Data is placed on the backplane as triggered by a first edge of the clock signal, and after a period of stabilization, read from the backplane on a second edge of the clock signal. The frequency of the clock signal will normally be adjusted to provide greatest data transfer rate commensurate with the limitations in the hardware in writing and reading the data. In such synchronous systems, at any given time, one modular device will have control of the backplane for transmitting data to prevent transmission conflicts.
As new I/O modules are developed having extended functions, pre-existing backplanes may have insufficient capacity to communicate the necessary data at the desired rate. While modification of the backplane to accommodate faster data transmission is possible, for example, by increasing the clock frequency, such modification risks losing compatibility with pre-existing "legacy" modules. Failure to maintain such compatibility makes using a new "extended function" module unattractive to the extent that it may require the customer to replace all legacy hardware. Additional costs may be incurred if rewiring I/O lines to modules replacing the legacy modules, and changing the control program to accommodate the replacement modules is required.